This application claims priority of Korean Patent Application 1999-49704, filed Nov. 10, 1999 and Korean Patent Application 2000-04587, filed Jan. 31, 2000.
1. Field of the Invention
The present invention relates generally to a squirrel cage rotor for induction motors, and more particularly to a squirrel cage rotor that has a polymer resin part containing powder of high magnetic permeability.
In addition, the present invention relates to a method of fabricating the squirrel cage rotor using jig so as to prevent the conductor bars from being buckled.
2. Description of the Prior Art
In a rotating body such as a rotor of a motor, a centrifugal force is generated in proportion to the mass of the rotor and the square of the rotational speed of the rotor, so that the rotor with high rotating speed may have, excessive deformation due to a large stress by the centrifugal force.
The quality of machined products mainly depend on the precision of machine tool which is equipped with built-in type spindle system that is composed of the rotor of motor and the spindle shaft, but the heavy mass of rotor can deteriorate dynamic stability of the built-in type spindle system because the heavy mass of rotor may cause excessive stress and deformation. Therefore, it is important to reduce the inertial force of the rotor by reducing mass of rotor.
The rotating speed of a built-in type spindle generally is restricted by a DN value (where D is a bearing nominal diameter in mm and N is the revolutions per minute of the spindle) and the critical whirling vibration frequency of the spindle. In particular, in the case of a built-in type spindle which is composed of the spindle shaft and the rotor of motor, the bending natural frequency is largely influenced by the mass of the rotor of a motor. Accordingly, as the mass of the rotor of a motor is increased, the bending natural frequency of a built-in type spindle is decreased, thus the heavy mass of rotor limits the maximum rotating speed of the built-in type spindle.
Accordingly, for a built-in type spindle rotating at high speed, it is necessary to increase the specific bending stiffness of the built-in type spindle shaft or decrease the mass of the rotor of a motor.
A conventional squirrel cage rotor is illustrated, in FIGS. 1 and 2.
As depicted in FIGS. 1 and 2, this conventional squirrel cage rotor includes a rotating shaft 11. A silicon steel rotor 12 is fitted around the rotating shaft 17. The silicon steel rotor 12 is fabricated by stacking a plurality of silicon steel sheets 14. Plurality of conductor bars 16 are respectively inserted into the holes 13 and then the two end rings 17 are fabricated by die-casting. Therefore, the conductor bars 16 are short-circuited by two end rings 17 that are positioned at both ends of the silicon steel rotor 12.
Since this conventional squirrel cage rotor has heavy mass due to the high density of silicon steel not only a large centrifugal force is generated inside of rotor body, but also is high torque is required to drive the motor.
In order to solve above problems, Korean Pat. Appln. No. 98-50187 discloses xe2x80x9cComposite squirrel cage rotor with magnetic powders and method for fabricating itxe2x80x9d.
With reference to FIGS. 3a and 4, the invention of this patent is described in detail.
As shown in FIGS. 3a and 4, a composite squirrel cage rotor 20 is composed of a composite pipe 22, a rotating shaft 21 and a squirrel cage conductor 23 which is positioned around the pipe 22 and provided with a plurality of slots 24. A plurality of heat pipes 25 may be inserted into the slots 24, respectively.
In this case, the pipe 22 is, fabricated by stacking and curing of composite prepregs which is composed of polymer resin and high strength and modulus fibers. In this case, the pipe 22 is made of fiber reinforced composite material whose polymer resin contains powder of high magnetic permeability such as iron powder in order to improve the performance of the motor.
As depicted in FIG. 3a, this squirrel cage conductor 23 is made of copper or aluminum having high electric conductivity. Plurality of slots 24 are machined along the axis of the squirrel cage conductor 23 by using an end mill or an electric discharge machine tool or a laser machine tool. Each of the slots 24 is formed to be straight along the axis or have a certain angle with the axis. The slots 24 are spaced apart from one another by predetermined regular intervals. In this case, conductor bars and two end rings are integrated into a single squirrel cage conductor 23.
However, this squirrel cage conductor 23 has a problem of high machining cost. In order to solve this problem, another composite squirrel cage rotor 30 is proposed as shown in FIGS. 5 and 6. This composite squirrel cage rotor 30 includes, two end rings 37. A plurality of holes are formed in each of the end rings 37 and the both ends of the same number of conductor bars 38 are inserted into holes of two end rings 37 by interference fit, with appropriate axial load.
The conductor bars 38 are made of copper or aluminum because copper and aluminum, have high electric conductivity. But since copper or aluminum has low stiffness, the conductor bars 38 which have small diameter relative to the axial length are easily buckled by an excessive axial load during the assembly of the squirrel cage conductor.
After the polymer resin part is cured, the squirrel cage conductor combined with the polymer resin part is ground in order to expose the small portion of the conductor bars 38 on its outer surface. If the conductor bars 38 have been buckled, the exposed areas of the conductor bars 38 are not uniform along the axis of the squirrel cage rotor. This may to cause the performance deterioration of the composite squirrel cage rotor.
Accordingly, the present invention has been considered the above problems occurring in the prior art, and an object of the present invention is to provide a squirrel cage rotor, which has a polymer resin part containing powder of high, magnetic permeability so that the rotor has low mass.
Another object of the present invention is to provide a squirrel cage rotor having a polymer resin part containing powder of high magnetic permeability, which is provided with a plurality of heat pipes to dissipate heat generated by induction operation between the stator and the rotor of motor.
A further object of the present invention is to provide a method for fabricating a squirrel cage conductor, which prevents conductor bars from being buckled.
In order to accomplish the above objects, the; present invention provides a squirrel cage rotor, comprising: a rotating shaft; a polymer resin part containing powder of high magnetic permeability; and a squirrel cage conductor made of material having high electric conductivity and positioned around the outer portion of the polymer resin part; wherein the powder of high magnetic permeability has a role of increasing the flux density of rotor and is uniformly distributed in the polymer resin part.
In accordance with a feature of the present invention, the slots of the squirrel cage conductor are provided with a plurality of heat pipes, respectively.
In accordance with a feature of the present invention, the rotor further comprises an inner core of high magnetic permeability so as to guide the magnetic flux from the stator to the rotor of motor.
In accordance with a feature of the present invention, chopped fibers are added to the polymer resin part so as to improve mechanical properties such as thermal stability and stiffness.
In addition, the present invention provides a method for fabricating a squirrel cage rotor, the rotor having a rotating shaft, a plurality of conductor bars, two end rings and a polymer resin part containing powder of high magnetic permeability, comprising the steps of: surrounding each of the conductor bars by a pair of jig; inserting the ends of the conductor bars into the holes of the end rings; removing the jig from each conductor bar; curing the polymer resin part containing powder of high magnetic permeability after inserting the squirrel cage conductor composed of the conductor bars and the end rings into the mold; and grinding of the outer surface of the composite squirrel cage rotor to composed of the conductor bars, the end rings and the polymer, resin part.
In accordance with a feature of the present invention, the jig is fabricated by axially dividing a cylinder into two equal parts, each of the conductor bars is surrounded by the jig, and the jig is shorter than each of the conductor bars in axial length.
In accordance with a feature of the present invention, the method further comprises the steps of fastening band clamp around the jigs surrounding the conductor bars, and removing the band clamp and the jigs from the conductor bars after the assembly of the conductor bars and the end rings.